It’s a routine new mothers know well: put the baby to sleep, hope the quiet will last, and then wake in the middle of the night to a chorus of wails and howls.

It’s not obvious why babies disrupt the peace so frequently, particularly at night. Maybe they’re hungry, hot, or just plain cranky—but maybe it’s more complex than all that.

Evolutionary biologist David Haig has a new idea about why babies cry at night. He theorizes that they cry in order to distract their mothers and prevent future pregnancies. Frequent nighttime nursing delays a woman’s fertility, too, and can precipitate hormone changes that curb ovulation.

Babies might cry in the middle of the night to prevent their mothers from getting pregnant again.

Not all scientists are convinced that Haig is on the right track. Some argue that babies rouse in the night for more immediate needs like food, water, or comfort. Self-initiated crying could also be a survival technique that prevents a baby from sleeping too soundly.

But if Haig is right, the little screamers are doing all they can to prevent another baby from coming along and ruining their good thing. That self-interest is in direct conflict with the mother’s evolutionary goal, which is to shove her genes into as many children as possible. These divergent goals, Haig says, are an overlooked part of child-parent relationships. “Mothers have evolved to maximize their numbers of surviving children, which is different from maximizing the survival of each individual child,” he says.

There’s no way to go back and test whether night nursing actually helped babies survive in the early chapters of our evolutionary history. Today’s babies are growing up in a world that doesn’t look much like the one in which this trick could have been useful. “I think that it’s an adaptation for a world very different from the current world,” Haig says. Contraception, solid nutrition and good health care have probably removed modern babies’ drive to prevent another sibling.

Haig says this means mothers can probably relax a bit and know that an unconsolable infant isn’t necessarily a desperate one—the tears might just be a behavioral remnant of the past. That doesn’t permit carelessness, of course, and mothers should still pay close attention to their baby’s needs. But they may rest a little easier knowing that nighttime crabbiness is pretty common. And with Mother’s Day on the way, that’s welcome news.

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